Modem Relay

Modem relay is functionally equivalent to fax relay except that modems are the end devices rather than fax machines. Voice gateways demodulate and modulate the modem signals as they enter and exit the IP network while the actual modem data is "relayed" across IP using special modem relay protocols.

From a Cisco voice gateway perspective, there are multiple modem relay implementations. Two of these implementations are Cisco proprietary, and although the other is standards based, it is not supported. Table 5-4 summarizes these different types of modem relay.

Table 5-4 Modem Relay Types

Modem Relay Type

Modulations Supported


Cisco modem relay

V.34 and V.90

Proprietary implementation using NSEs to signal the switchover. V.90 modulations are forced down to V.34 speeds.

Secure modem relay (secure communication between STE endpoints)

V.32 and V.34

Designed to support line-side, trunk-side, and IP secure terminal equipment (STE) endpoints. Requires SCCP/ MGCP gateways and Unified CallManager. Uses standards-based V.150.1-based SSE (state signaling events) messages for the switchover, but other protocol aspects are Cisco proprietary.

ITU-T V.150.1 modem relay

V.92, V.90, V.34, V.32bis, V.32, V.22bis, V.22, V.23, and V.21 when acting as Universal-Modem Relay gateway and V.8 negotiated modulations for V.8-Modem Relay gateways

Standards-based modem relay designed for multivendor interoperability. Uses an SSE switchover mechanism and is not supported on Cisco gateways.

All the preceding modem relay types transfer the demodulated modem data across the IP network using some form of SPRT (Simple Packet Relay Transport). Formally defined in Annex B of the ITU-T V.150.1 specification, SPRT is a low-overhead, reliable protocol running over UDP/IP. Figure 5-16 highlights the basic SPRT frame format as defined by ITU-T V.150.1.

NOTE The Cisco proprietary modem relay methods might not implement this exact SPRT packet format, but it is still shown for reference to provide a glimpse of the underlying modem relay transport protocol.

Figure 5-16 ITU-T V. 150.1 SPRTPacket Format

Figure 5-16 ITU-T V. 150.1 SPRTPacket Format

X: Header Extension Bit - set to 0, reserved for ITU-T.

SSID: SubSession ID - identifies a SPRT transmitter subsession.

PT: Payload Type - value assigned by external call signaling upon call setup.

TC: Transport Channel ID - indicates sequencing and reliability parameters as defined in Table B.1 of ITU-T V. 150.1.

Sequence Number: Used by SPRT transmitter for packet sequencing when required.

NOA: Number of Acknowledgments - specifies number of ACK fields in SPRT header.

Base Sequence Number: Identifies the sequence number of the next packet that will be received for the specified TC.

TCN, SQN: ACK indication fields, up to 3 as specified by NOA, TCN identifies the Transport Channel ID for the proceeding SQN (Sequence Number) value.

Because standards-based modem relay is not currently supported on Cisco voice gateways, a detailed, technical discussion of V.150.1 is not pertinent to this section. Therefore, the remainder of this section focuses on Cisco modem relay, the most popular modem relay implementation for Cisco voice gateways. Additional information about secure modem relay is provided in the section "Secure Modem Relay" in Chapter 7.

Most modem connections today negotiate the use of an error correction (EC) protocol. These EC protocols typically introduce some sort of synchronous framing for the modem call so that asynchronous frames are no longer needed on the connection between the modems.

For modem relay, a synchronous frame structure is mandatory for efficiently transporting the modem data. Although other modem relay implementations may support multiple EC protocols, Cisco modem relay takes advantage of the Link Access Procedure for Modems (LAPM) framing specified by the V.42 EC protocol. For more information on the V.42 EC protocol and LAPM framing, see the section "Error Control" in Chapter 1, "How Modems Work."

In a traditional modem-to-modem V.42 connection, the connection parameter negotiation, which includes items such as window and frame size, is handled by the modems themselves through XID (Exchange Identification) frames. However, with Cisco modem relay, the V.42 negotiation is handled independently by the voice gateways. Figure 5-17 depicts the interaction of the V.42 protocol with Cisco modem relay.

Figure 5-17 ITU-T V.42/V.42bis Negotiation Within Cisco Modem Relay

V.42bis is negotiated locally but settings are i matched end-to-end. i

i V.42bis i

Cisco Modem Relay i i

Cisco Modem Relay i i

The V.42 negotiation sets up the synchronous, link layer connection. Parameters such as window size and frame size are specified. These parameters might not necessarily match between each voice gateway and modem because of the independent negotiation that occurs between each modem and gateway pair.

V.42bis details the data compression procedures used in conjunction with V.42. Cisco modem relay supports compression in only a single direction, both directions, or compression can be disabled altogether.

When V.42bis compression is present, the modems themselves handle the compression and decompression functions. V.42bis is not terminated locally by the gateways like V.42 is. The Cisco voice gateways just ensure that the V.42bis parameters between the modems are synchronized. See the section "Data Compression" in Chapter 1 if you need more detailed information about V.42bis.

The only switchover mechanism available for Cisco modem relay uses NSE packets. Like the other NSE switchover methods, this procedure is call signaling protocol independent and works the same for H.323, SIP, MGCP, and SCCP. Figure 5-18 details the NSE switchover for Cisco modem relay.

Two specific NSE messages are associated with Cisco modem relay. The first is an NSE-199, which allows the gateways to inform each other that they support Cisco modem relay. This NSE message is sent out by the terminating gateway as soon as the ANSam tone has been detected and the NSE-192 has been sent to trigger modem passthrough.

Until the gateways are sure that this VoIP call is a modem call that can be supported by Cisco modem relay, there cannot be a formal switchover to modem relay. For example, after only the ANSam tone has been heard, the call could end up being an SG3 fax call. An SG3 fax call is not compatible with Cisco modem relay, but it looks the same at this stage of the call.

The second NSE message used in Cisco modem relay is the NSE-203. This NSE message forces the Cisco modem relay switchover assuming that the NSE-199 messages have been properly exchanged. The NSE-203 message is only triggered by the detection of a valid V.8 CM (Calling Menu) by the OGW. After the appropriate V.8 CM has been detected, the gateways are sure that this is a supportable modem call, and the modem relay feature can be invoked.

Figure 5-18 Cisco Modem Relay Switchover Using NSE Packets

Figure 5-18 Cisco Modem Relay Switchover Using NSE Packets

TIP The switchover implementation of Cisco modem relay discussed above is referred to as gateway controlled or gw-controlled in the Cisco IOS gateway CLI. Introduced in Cisco IOS Release 12.4(4)T, this implementation requires no work on the part of the call signaling protocol with regard to the switchover process.

In earlier codes, a method known as signaling-assisted modem relay was used. Instead of exchanging NSE-199 messages, this switchover method depended on information exchanged within the voice signaling protocol to confirm that both sides were capable of Cisco modem relay. However, NSE-203 messages were still used to trigger the actual switchover.

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